round() does not return a string, but a floating point number, the best
approximation to the "real" result that is achievable with the limited
precision of binary floating point.

That result is then formatted to a string for display by the interactive
interpreter using the repr() function, which formats a float with (I
think) 17 decimal digits.

So, the point here is that repr() produces "too many" digits for your
liking ;) You can use str() to cut off at fewer digits, which will
produce the "correct" string in most cases.